(a) Field of the Invention
The present invention relates to working plasma torch with ignition by short-circuit. It is firstly concerned with an electrode for such a torch, of the type comprising a support of electroconductive material having an end provided with an emissive insert, such as of zirconium or hafnium.
(b) Description of Prior Art
Cutting plasma torches operating with a plasma producing oxidizing gas are generally provided with a zirconium or hafnium emissive cathode, which is inserted in a copper member constituting an electroconductive support and a coolant, the assembly being generally called electrode.
During the operation of the torch the root of the arc is caught in the center of the cathode, where a crater is progressively formed on whose surface there is produced a layer of an oxide of zirconium or hafnium. When hot, this oxide is fluid and conductive and permits cathodic emission. When the plasma arc is interrupted, the oxide solidifies and becomes insulating. During solidification, there is frequently formed in the periphery of the crater, a swelling of oxide which projects on the front surface of the insert.
In the known arrangements, the insert is flush with the end face of the support. Then, the oxide swelling projects beyond this end face. If the swelling constitutes no impediment in the case of a conventional torch, where electrode and tuyere are fixed and where the formation of the arc is inititated by a high frequency discharge, in the case of short-circuit ignition torches such as those described in FR-A-2 556,549 and 2 562,748, where the ignition of the arc is obtained by producing a short-circuit between the electrode and the tuyere, it so happens that this oxide swelling forms an electrically insulating barrier which prevents the reliable passage of current between the electrode and the tuyere, and consequently, the reliable ignition of the arc.